The present invention relates to an optical harmonic generator for converting the characteristics of an optical beam using the nonlinear optical effect of a substance, and more particularly to an optical harmonic generator which can stably produce harmonics based on optical harmonic generation, parametric excitation, or the like.
Heretofore, it is known that harmonics such as a second harmonic can be generated on the principle that when a laser beam is applied to a nonlinear optical element, the wavelength of a laser beam emitted from the nonlinear optical element is reduced by one half. It is now assumed that the laser beam applied to the nonlinear optical element is referred to as an .omega. wave having an angular frequency of .omega. and the laser beam emitted from the nonlinear optical element is referred to as a 2.omega. wave, i.e., a second harmonic, having an angular frequency 2.omega..
One conventional harmonic generator in the form of a 1/2-wavelength light generator is illustrated in FIG. 9 of the accompanying drawings.
The 1/2-wavelength light generator comprises a semiconductor laser 1 for emitting a laser beam (.omega. wave) having an angular frequency of .omega., a frequency converter 2 for applying the laser beam (.omega. wave) to a nonlinear optical crystal to cause the nonlinear optical crystal to generate a laser beam (2.omega. wave) having an angular frequency of 2.omega., a condensing lens 8 for converging the .omega. wave from the semiconductor laser 1 onto the center of the nonlinear optical crystal of the frequency converter 2, a semiconductor laser driver power supply 9 for supplying a drive current to the semiconductor laser 1, a semiconductor laser temperature regulator 51 for regulating the temperature of the semiconductor laser 1 at a constant level, and a crystal temperature regulator 10 for regulating the temperature of the nonlinear optical crystal of the frequency converter 2 at a constant level.
The .omega. wave applied to the nonlinear optical crystal of the frequency converter 2 may be an infrared radiation having a wavelength of 860 [nm], for example, and the 2.omega. wave emitted from the nonlinear optical crystal through frequency conversion may be blue light having a wavelength of 430 [nm], for example. If the nonlinear optical crystal is of potassium niobate, its temperature is maintained at a constant 25.degree. C. by the crystal temperature regulator 10.
Operation of the conventional 1/2-wavelength light generator shown in FIG. 9 will be described below.
A constant drive current I.sub.1 is supplied from the semiconductor laser driver power supply 9 to the semiconductor laser 1, which then emits an .omega. wave having a frequency of .omega. corresponding to the drive current I.sub.1. The .omega. wave thus emitted is converged by the condensing lens 8 and applied as a convergent laser beam to the center of the nonlinear optical crystal of the frequency converter 2.
In the nonlinear optical crystal of the frequency converter 2, the introduced .omega. wave is increased in energy density as a convergent Gaussian beam, and converted in frequency into a 2.omega. wave with high efficiency. In order to effect efficient generation of the 2.omega. wave with high efficiency, it is necessary that the .omega. wave and the 2.omega. wave be brought into phase with each other, i.e., that so-called phase matching be achieved, in the nonlinear optical crystal. The condition for achieving such phase matching is given by the following equation: EQU sin.sup.2 .theta..sub.m ={(n.sub.o.sup..omega.).sup.-2 -(n.sub.o.sup.2.omega.).sup.-2 }/{(n.sub.e.sup.2.omega.).sup.-2 -(n.sub.o.sup.2.omega.).sup.-2 } (1)
where n.sub.o is a refractive index with respect to ordinary rays of light, n.sub.e is a refractive index with respect to extraordinary rays of light, .omega. and 2.omega. are angular frequencies, and .theta..sub.m is an angle at which the nonlinear optical crystal is cut in order to obtain a phase matching angle.
To achieve the phase matching based on birefringence of the nonlinear optical crystal in order to satisfy the phase matching condition expressed by the equation (1) mentioned above, the temperature of the nonlinear optical crystal is kept at a constant value by the crystal temperature regulator 10 so as to enable the nonlinear optical crystal to maintain a desired refractive index. Since the .omega. wave generated by the semiconductor laser 1 varies in wavelength depending on its temperature, the temperature of the semiconductor laser 1 is also maintained at a constant level by the semiconductor laser temperature regulator 51.
When the wavelength of the .omega. wave generated by the semiconductor laser 1 is thus maintained at a constant level and the refractive index of the nonlinear optical crystal of the frequency converter 2 is thus kept at a constant value, the phase matching condition of the 1/2-wavelength light generator is satisfied, making it possible to effect second harmonic generation (SHG).
If the wavelength of the .omega. wave changes to another wavelength, then since there is a different nonlinear optical crystal temperature corresponding to that other wavelength, it is possible to achieve phase matching at the different nonlinear optical crystal temperature. However, even if the wavelength of the .omega. wave changes to not only a longer wavelength but also a shorter wavelength, the output power of the 2.omega. wave is reduced in the same manner, making it impossible to determine a direction in which the temperature should vary. To avoid the above drawback, the temperature of the semiconductor laser 1 and the value of the drive current have to be kept constant to maintain the wavelength at a constant level, and at the same time it is necessary to maintain the temperature of the nonlinear optical crystal at a constant value for phase matching.
For maximum frequency conversion efficiency, the semiconductor laser 1 and the nonlinear optical crystal of the frequency converter 2 need to be associated with the respective temperature regulators 51, 10. Therefore, the entire device arrangement is complex and large in size. Since all of the temperature of the nonlinear optical crystal of the frequency converter 2, the temperature of the semiconductor laser 1 and the drive current of the semiconductor laser 1 are held constant during operation, when the wavelength of the laser beam emitted from the semiconductor laser 1 varies with respect to the supplied drive current due to aging, for example, the efficiency with which the 2.omega. wave generates is lowered by a degree which corresponds to the wavelength change. Accordingly, the conventional 1/2-wavelength light modulator is unable to operate stably over a long period of time.